The present invention relates to a polypropylene resin composition and, more particularly, to a polypropylene resin composition which is composed mainly of a crystalline ethylene-propylene copolymer, which is well balanced particularly in rigidity and impact resistance, has good mar resistance and excellent moldability and can provide a molded article of excellent appearance. The polypropylene resin composition can be utilized in the field of large-sized parts for automobiles.
Crystalline polypropylene is in use in various molded articles in which physical properties such as rigidity, resistance to heat deformation and the like are required. However, having insufficient low temperature properties and being a non-polar highly-crystalline polymer, crystalline polypropylene has poor impact resistance and cannot be used in applications in which rigidity, resistance to heat deformation and impact resistance are required together.
In order to improve the impact resistance of crystalline polypropylene, there have been proposed, for example, a polypropylene composition which is a blend of a crystalline ethylene-propylene block copolymer as a crystalline polypropylene with an amorphous ethylene-propylene copolymer, and a polypropylene composition which is a blend of crystalline ethylene-propylene block copolymer, an amorphous ethylene-propylene copolymer and various polyethylenes.
In these polypropylene compositions, however, the rigidity, resistance to heat deformation, etc. inherently possessed by crystalline ethylene-propylene block copolymers are reduced.
In order to improve these drawbacks of the above polypropylene compositions, various attempts have been made. For example, Japanese Patent Application Kokai (Laid-Open) No. 64257/1978 discloses a resin composition with improved low temperature impact resistance, comprising a crystalline ethylene-propylene block copolymer, an amorphous ethylene-.alpha. -olefin copolymer and talc. The document describes that the resin composition, as compared with conventional crystalline ethylene-propylene block copolymers, is substantially improved in low temperature impact resistance, rigidity and coatability, and provides Examples for supporting the description. The above composition, however, is insufficient in rigidity-impact resistance balance, mar resistance and molded article appearance.
There have also been disclosed various polypropylene compositions comprising, as a filler other than talc, a granular filler (e.g. calcium carbonate) or a fibrous filler (e.g. glass fiber). However, in the polypropylene compositions comprising a granular filler, the molded articles have no sufficient rigidity; and in the polypropylene compositions comprising a fibrous filler, although the rigidity is improved significantly, the impact resistance is low and the molded articles have poor surface appearance and give rise to warpage, deformation, etc. For example, Japanese Patent Application Kokai (Laid-Open) No. 91545/1987 proposes a polyolefin resin composition comprising a polyolefin, a thermoplastic elastomer, a fibrous reinforcing agent, talc and calcium carbonate. This composition, however, tends to yield poor appearance (e.g. silver) during injection molding and has a small improvement in rigidity because it contains calcium carbonate. Japanese Patent Application Kokai (Laid-Open) No. 69848/1986 proposes a polypropylene resin composition comprising a polypropylene resin, a fibrous filler, talc and an elastomer. This composition, however, causes deformation when molded into large-sized parts for automobiles because it contains a large amount of a fibrous filler. Meanwhile, this composition gives a small improvement in rigidity when the content of fibrous filler is low.
Thus, the molded articles made from the hitherto known resin compositions cannot be used in, for example, large-sized parts for automobiles, particularly instrument panels in which not only mechanical properties but also appearance are important, even though they can be used in certain applications in which appearance is negligible.